One type of known cistern outlet valve is termed a buoyant outlet valve (sometimes referred to as a float valve). A buoyant outlet valve requires a very close tolerance (ie between the housing that fits over the top of the float and the guide shaft that runs through this housing and constrains the movement of the float). The close tolerance is necessary in order to restrict any water incident on the housing from flowing onto the top of the float. Any such water on the float places a positive pressure on the top face of the float and can result in the valve closing prematurely or at a higher level than is desired.
A disadvantage with valves of this type is that the close tolerance required between the housing and the float shaft is expensive and difficult to produce and does not provide a very efficient fluid seal. Further, the housing and the shaft are generally moulded from plastic materials and small moulding variations can lead to jamming of the valve, poor valve performance or premature wear between the contacting faces.